Hydrogen energy

Summary: Hydrogen could be the clean energy carrier of the future.

Most of the energy demand today is for fossil fuels. It is estimated that the global distribution of energy use is 80 percent fossil fuels, 14 percent renewable energy, and 6 percent nuclear power. The input of renewable energy sources such as solar, wind, and hydropower to the global energy grid is only 3 percent. The combustion of fossil fuels leads to the emission of great amounts of greenhouse gases. Some studies predict that global energy consumption will triple by about 2040. The environmental impact will be significant if fossil fuels continue to be used at high rates. The use of clean energy is becoming an urgent matter as the energy the world requires keeps increasing with the continuous rise in energy use by developing countries as their gross domestic product (GDP) numbers increase. Hydrogen could become a clean energy carrier, as it has a very important benefit: the absence of carbon dioxide from its combustion:

H2 + 1/2O2 g H2O

Energy from hydrogen combustion is 2.5 times higher than the energy released by the combustion of any other fuel, and it can be used in almost every application where fossil fuels are used. It can be said that H2 may be the energy carrier of the next several generations.

Hydrogen Production

The word hydrogen emanates from the combination of the Greek words hydro (water) and gene (generate). Hydrogen (H2) can be found almost in every compound, from water to hydrocarbons, but it has the “disadvantage” that it rarely occurs freely in its pure form. Furthermore, H2 needs to be generated. Since the 16th century, when H2 was first produced in the laboratory, a lot of progress has been made. H2 can be generated from a variety of feedstocks, including not only fossil fuels but also renewable energy sources such as biomass and water, utilizing several H2 production technologies. Several chemical, biological, electrolytic, and thermochemical technologies can be used, each of which offers unique opportunities, benefits, and challenges.

Hydrogen Production Technologies

Reformation is the most widespread and commercially applicable method of H2 generation globally, as it accounts for almost 97 percent of total H2 production. The reformation general reaction is expressed by CnHm + nH2O + Heat g nCO + (n + m/2) H2. Hydrocarbons react with steam, producing a gas mixture of carbon monoxide and hydrogen known as syngas (for synthetic gas). Carbon monoxide can be further treated through a water-gas shift reaction to produce hydrogen and carbon dioxide (CO2). The H2 content depends on the hydrocarbon feedstock as well as the process.

The principal feedstock used is methane (via steam methane reforming, SMR), which is the dominant component of natural gas (constituting between 87 and 96 percent). The main reformation stages are the feedstock purification, the steam reaction, and the first-stage cleaning, where CO is transformed into CO2. Reformation has several advantages, such as low energy requirements, good efficiency (65 to 70 percent), and low production costs, but it has one great deficit: the generation of CO2, which is the world’s major greenhouse gas (GHG). Nevertheless, further treatment of the gas mixture through carbon sequestration techniques provides clean hydrogen. Similar to reformation is partial oxidation, whereby hydrogen is produced through the partial combustion of methane with oxygen gas to yield carbon monoxide and hydrogen.

Hydrogen can be generated from coal through several gasification processes, producing syngas. Gasification is the reformation process when it is applied in solid hydrocarbons. Usual gasification feedstocks are lignite and biomass. The technology is quite mature, but its complexity and its production cost will not make it a viable option, at least in the near term.

Combustion of H2 with O2 produces water and energy. It is obvious that if this reaction is reversed, hydrogen could be produced from water. This is the idea of water electrolysis. Electrolysis uses direct electric current (DC) to drive a nonspontaneous chemical reaction in order to decompose water into its components, H2O + Electricity g H2 + 1/2O2. Alkaline water electrolysis, high-temperature electrolysis, high-pressure electrolysis, and polymer electrolyte membrane electrolysis are the basic electrolysis techniques.

Electrolysis is a quite mature technology and is currently the only practical technology to produce hydrogen from renewable energy sources. It is relatively efficient (greater than 70 percent), it does not require fossil fuels, and thus it has no environmental impact and produces a very clean H2. On the contrary, it is still not economically feasible, as it requires large amounts of electricity.

Besides electrolysis, water can be utilized via several routes in order to produce hydrogen. Photoelectrolysis, biophotolysis, and high-temperature water decomposition are some of the existing technologies, which are still in their research stages. They mostly require large amounts of heat, which is usually provided by nuclear energy.

Hydrogen Storage and Distribution

Hydrogen can be stored in all forms: gaseous, liquid, and, through chemical reaction, in a “solid” form. The most common method to store hydrogen is in gas form, in high-pressure tanks. The tanks’ storage pressure is usually at 20 megapascals, but it can reach even 80 megapascals based on the type of tank and material of construction. The tanks are usually constructed using steel, aluminum, carbon fibers, or a combination of these materials. Liquid hydrogen storage is another familiar method. Hydrogen is stored in spherical tanks at low temperatures (less than 53 degrees Celsius), and its pressure cannot exceed 10 megapascals. Hydrogen can also be stored in solid materials. Metal and chemical hydrides, under great pressure and at medium temperatures, have the ability to absorb hydrogen. “Solid” hydrogen is a relatively new H2 storage method with great potential, as it can be a safe and efficient way to store and distribute energy. Hydrogen can be transported using all the known distribution methods. Liquid hydrogen can be transported in special hydrogen tankers as liquid fuel, while gaseous hydrogen can be transported as natural gas through pipelines from the production point to the consumption point. Currently, there are 450 miles (725 kilometers) of pipelines in the United States and a 248-foot (400-kilometer) distribution network in the European Union, connecting France and Belgium.

Hydrogen Use

Hydrogen has numerous applications. It is used in massive quantities in the petroleum and chemical industries, in physics and engineering, and even in the food industry. Hydrogen is the lightest element in the periodic table. This property makes hydrogen ideal as a lifting agent. In the 1930s, Germany constructed airships called zeppelins to transport people and goods across Europe. By that time, helium, another lighter-than-air gas, was quite expensive to obtain, and so hydrogen was used. Unfortunately, hydrogen is quite flammable. In 1937, while the zeppelin Hindenburg was trying to land in Lakehurst, New Jersey, it suddenly burst into flames, causing the deaths of 36 people. After that, the idea of using hydrogen as a lifting agent was abandoned.

On the other hand, the flammability of H2 and its energy content make it ideal for fuel. It can be used in power plants and in the transportation sector. Hydrogen can be the driving force of cars and buses in two ways: via direct combustion and through its utilization in fuel cells. Ford Motor Company introduced the car model P2000, which uses hydrogen instead of gasoline, and Honda introduced the car model FCX, which is powered by fuel cells.

In aviation, hydrogen has great potential as well. In the 1950s, the B27 was running on liquid hydrogen, and in 1988, the TU 155 proved the feasibility of a passenger aircraft being powered by liquid hydrogen. CRYOPLANE, a European research consortium, conducted a systems analysis of hydrogen as an aviation fuel. It considered applicability, safety, and environmental issues. The main findings of the technical assessment were that hydrogen is a suitable alternative to kerosene (used in aircraft), there are no barriers to the implementation of its use, and there are great long-term benefits.

Hydrogen also holds great promise for use in fuel cells, devices that convert electrochemical energy. Fuel cells are fueled by hydrogen and oxygen to produce electricity, heat, and water. They are made up of three segments: the anode, the electrolyte, and the cathode. Hydrogen is fed to the anode and oxygen to the cathode. Activated by a catalyst, hydrogen atoms separate into protons and electrons, which, with an external circuit, create electricity. Currently, six major fuel cell technologies are under development: polymer electrolyte membrane (PEM), phosphoric acid, direct methanol, alkaline, molten carbonate, and solid oxide, which is the dominant one. Fuel cell technology has great potential, as it can find applications in all areas, from small mobile electric devices, such as cell phones, to cars such as the Honda FCX.

Hydrogen as a fuel has a great advantage over hydrocarbons. Its combustion and its use in fuel cells have a negligible environmental impact. It is obvious that its environmental performance depends on its production technology.

The principal hydrogen generation technologies are the reformation of hydrocarbons and water electrolysis. Reformation generates CO2 as a byproduct. In addition, the electricity required for electrolysis has its own environmental burden. Hydrogen has great potential to be a clean energy carrier if a way is found to overcome these constraints. Carbon sequestration technologies hold the potential to minimize CO2 production, and renewable energy sources are generating clean electricity. The combination of these new technologies could give life to the future of clean hydrogen production.

Bibliography

“Corporate Document Repository: Chapter 5. Hydrogen Production.” Food and Agriculture Organization, www.fao.org/docrep/w7241e/w7241e0g.htm. Accessed 2 Aug. 2024.

Hirscher, Michael, and Katsuhiko Hirose. Handbook of Hydrogen Storage. Stuttgart, Germany: Wiley VCH, 2010.

“What Is Hydrogen?” National Grid, 23 Feb. 2023, www.nationalgrid.com/stories/energy-explained/what-is-hydrogen. Accessed 2 Aug. 2024.